1. Field of the Invention
The present invention relates to data transmission in general, and more particularly to a method of allocating transmission resources in an asynchronous transfer mode communications system.
2. Description of the Related Art
There are already known various constructions of telecommunications systems, among them such which employ a so-called asynchronous transfer mode in operation. Systems of the latter type typically include a central unit and a plurality of separate terminals that communicate either with the central unit or with one another, but always under the control of the central unit. This control is necessary in such an asynchronous system, in that the central unit has to allocate transmission resources (also known as time slots) for contents transmission to the individual terminals so that they would operate sequentially rather than simultaneously. The communication may take place, for instance, through the airwaves (radio transmission), and the terminals may include, for instance, radio telephones, computers or other similar communications devices. However, at least some of such terminals could be connected to the central unit (or to one another as well) by different media, such as through electrical, optical or other cables. Be it as it may, the central unit always has to coordinate the operation (timely transmission) of these terminals.
In the heretofore proposed arrangements (and associated methods) of this kind, the individual terminals inform the central unit about the status of their buffers, especially the filling status of their output and input buffers, and the central unit allocates the transmission resources to such terminals on the basis of this information. The allocation of the transmission capacity in a communication system of this kind is revealed, for instance, in the German patent application 197 26 120.5, in the article by D. Petras and A. Krxc3xa4mling xe2x80x9cMAC Protocol with Polling and Fast Collision Resolution fir an ATM Air Interfacexe2x80x9d, IEEE ATM Workshop, San Francisco, Calif., August 1996, or in the article by D. Petras, A. Krxc3xa4mling and A. Hettich xe2x80x9cMAC Protocol for Wireless ATM: Contention Free versus Contention Based Transmission of Reservation Requestsxe2x80x9d, PIMRC ""96 Taipei, Taiwan, October 1996.
In these and similar applications, a MAC (Medium Access Control) channel capture protocol is typically used. For the terminal to request the resources. It transmits a so-called resource request (RR) data string, in which it informs the central unit about its needs. These RR data strings are valid for an individual DLC (Data Link Control) connection, and include the information about the number of the data strings or packets that are waiting in the buffers of the respective terminal for transmission. In this respect, it is important to realize that the number of such waiting data packets that are temporarily stored in the respective buffer may greatly exceed the transmission capacity of a single MAC transmission frame. The central unit allocates the capacities of the individual phases of a MAC transmission frame, in turn, on the basis of the individual DLC connections. For the downlink phase (i.e. central unit to terminals), the central unit announces which one of the terminals is to receive which information. For the uplink phase (terminal to central unit), it is announced which one of the terminals is permitted to send for a DLC connection at a particular time. For the Direct Mode Channel (direct communication between the terminals), the sender and receiver and the DLC connection are announced.
In this respect, it is to be borne in mind that the RR data ought to indicate the actual filling status of the buffer under all circumstances. In order to have the transmission capacity allocated to the respective terminal by the central unit on the basis of the filling status, it is not advisable for the respective terminal to report to the central unit anything else but the actual filling status of its buffer, inasmuch as the urgency of the transmission could otherwise by judged erroneously under certain circumstances, were it otherwise.
The mechanisms for requesting and allocating the transmission resources that have been mentioned above are particularly, but not exclusively, suited for the burst-type, i.e. statistics-based, telecommunication traffic. However, there is also another type of data transmission (referred to herein as deterministic) which is more or less constant, i.e. the amount of data transmitted per unit of time does not vary to any great extent, if at all. The possibly most widespread example of such a deterministic traffic, which is also often referred to as CBR (Constant Bit Rate) traffic, is certainly voice communication without compression, in which, generally speaking, 64 Kbit/s are generated with great uniformity. These data are then buffered at the transmitter until, for instance, an ATM cell is filled. These full ATM cells are then periodically transmitted into the communications system.
In the course of ATM signalization, the parameter data rate, intermediate arrival time and the maximum variation (jitter) in the intermediate arrival time are transmitted in the course of initially establishing the connection.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a method of allocating transmission resources in a centrally controlled communications system that does not possess the drawbacks of the known methods of this type.
Still another object of the present invention is to devise a method of the type here under consideration which would be particularly suited for use in conjunction with transmitting, besides statistical traffic, also deterministic traffic.
It is yet another object of the present invention to design the above method in such a manner as to be able to simplify the exchange of control information between the central unit and the terminals communicating with it.
A concomitant object of the present invention is so to construct the equipment for performing the method of the above type as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.
In keeping with the above objects and others which will become apparent hereafter, one feature of the present invention resides in a method of allocating transmission resources to respective terminals in a centrally controlled communication system in which the transmission resource allocation to the terminal is performed by a central unit; this method includes allocating the transmission resources for at least one frame-oriented connection of a deterministic kind, particularly such that operates with a substantially constant data rate, independently of any previously issued resource requests for non-deterministic connections; providing a special designation for such a deterministic connection to distinguish the same from other connections; and correcting non-synchronous timing between the central unit and the affected terminals particularly for such a deterministic connection.
A particular advantage of the method as described so far is that it renders it possible to optimize the allocation of the transmission resources, especially when connection with substantially constant data transmission rate are present in the system. In accordance with the present invention, the otherwise existing need for explicit resource requests issued by the terminals operating with the constant data transmission rate can be dispensed with, In this manner, the processing in the central unit is simplified, and the resource capacity is saved.
The present invention is advantageously particularly suited for use in such communications systems, in which in addition to the data-burst traffic, there is also encountered deterministic traffic, that is traffic in which particularly the data amount per unit of time (data rate) is substantially constant, or in which the intermediate arrival time between the successive data packets is substantially constant. In the ATM (Asynchronous Transfer Mode) transmission scheme, this type of traffic or communication is often referred to as a Constant Bit Rate (CBR) traffic. However, the present invention can also be used in different environments, that is, independently of ATM, for all deterministic kinds of traffic.
In heretofore proposed or constructed systems, for instance in the actual concept employed in the Hyperlan Type 2 communication system, transmission resources are allocated by the central unit only when they have been specifically previously requested by the respective terminal. Yet, the additional information that is available to the central unit about such CBR connections is not utilized. In contradistinction to this, the present invention utilizes this additional information in order to optimize the process of resource allocation. Even though, in accordance with the present invention, the transmission resource allocation for the connections with the generally constant data rate is accomplished without any prior request to the central unit for such allocation, the central unit nevertheless allocates the requisite transmission resources to the affected terminals in amounts resulting from, in each instance, the applicable data rate as it became known during the initial establishment of the respective deterministic connection with the respective terminal, or the intermediate arrival time of the data packets (cells). Simultaneously, corrections are made to eliminate the effect of non-synchronous operation of the respective terminals relative to the central unit and/or one another.
Advantageously, the allocating step includes transmitting a connection characteristic for the deterministic connection in the course of the establishment of this connection, and setting the resource allocation for the respective terminal on the basis of this connection characteristic. The allocating step advantageously includes indicating a data rate for the deterministic connection in the course of establishment of this connection. This indicating step may include expressing the data rate as a number of data packets of constant length that are to be transmitted within a predetermined number of frames, or as a data capacity to be transmitted within a predetermined time interval.
According to another aspect of the present invention, the correcting step includes inserting filler packets devoid of useful information into the respective frame when the actual data packets are too slow in arriving. On the other hand, the correcting step includes issuing resource requests for additional allocation when the actual data packets arrive at too rapid a pace. The correcting step advantageously further includes granting such resource requests for the deterministic connection independently of any other transmission resource requests.
Another facet of the present invention is to be seen in the fact that the inventive method comprises the step of performing the allocating, providing and correcting steps during an uplink phase as the respective terminal is to transmit information to at least one of the central unit and another terminal. This method may advantageously further include the step of additionally allocating the resources remaining after the performance of the allocating step to any connection of a non-deterministic nature in dependence on the buffer filling status encountered at the respective terminal associated with this particular non-deterministic connection. The allocating step advantageously includes transmitting and evaluating respective data rate, intermediate arrival time of data packets, and maximum deviation of the data packet arrival rate parameters in establishing the non-deterministic connection characteristic. Last but not least, it is advantageous the allocating step includes utilizing additional information that is available for the respective deterministic connection at the central unit, such as the expected arrival of the next following data packet, for the optimization of the resource allocations.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.